The use of digital data communication has become widespread to the point of nearly being ubiquitous. For example, digital communications are routinely implemented in providing data communications in various systems, such as computer network systems (including personal area networks (PANs), local area networks (LANs), metropolitan area networks (MANs), wide area networks (WANs), the Internet, etc.) and communication systems (including the public switched telephone network (PSTN), cellular networks, cable transmission systems, etc.).
In operation of such digital data communications, binary information is transmitted from a transmitter, through a transmission medium, to one or more receivers. For example, a data source at the transmitter may provide a signal (e.g., a voice signal, a video signal, a multimedia signal, etc.) that is digitized and then modulated by varying one or more parameters (e.g., frequency, phase, or amplitude) of a particular carrier wave (e.g., a square wave or sinusoidal wave), whereby the modulated carrier wave is transmitted into the transmission medium. Non-coherent modulation techniques, such as frequency-shift keying (FSK), and differentially coherent modulation techniques, such as differential phase-shift keying (DPSK), are used for digital data communications in many applications including code division multiple access (CDMA) communication systems. The receiver, upon receiving the modulated carrier wave via the transmission medium, demodulates and decodes the signal. However, the transmitted signal may be corrupted or otherwise degraded by noise, interference, and/or distortion in the transmission medium. Accordingly, the receiver demodulates and decodes the degraded signal, as received by the receiver, to make an estimate of the original data.
Further complicating the ability of the receiver to accurately estimate the original data from the received degraded signal is that the transmitter and receiver are typically not synchronized. For example, clocks implemented by the transmitter and receiver with respect to the carrier wave may not be synchronized, propagation delays may introduce timing differences with respect to the signals at the transmitter and receiver, etc., thereby presenting issues with respect to timing synchronization. Similarly, manufacturing differences with respect to crystals, mixers, and other circuitry may introduce frequency differences presenting issues with respect to frequency synchronization. Accordingly, synchronization of the received signal at the receiver is often important, whereby timing, phase, and/or frequency synchronization is performed in order to accurately retrieve the original data. In packet-based communication systems in particular, time and frequency synchronization is often critical for efficient transmission, wherein time synchronization determines the boundary of a symbol of the received signal and frequency synchronization estimates and compensates for the carrier frequency offset (CFO) between the transmitter and receiver.
In performing digital communications with a transmitter, a receiver may thus operate to interpret a received signal for establishing timing and frequency synchronization with the transmitter. For example, when a user switches on a mobile cellular phone to communicate with a base station, the mobile phone receiver may operate to synchronize its timing and the frequency with the base station transmitter. The synchronization functions of the receiver are thus generally tied to the received signal whereby the synchronization information is derived from the received signal.
A number of techniques for time and frequency synchronization have been utilized with respect to FSK and DPSK modulated signals. However, these techniques have generally not been well suited for use with respect to some communication scenarios or communication system configurations. For example, the existing time and frequency synchronization techniques are often not compatible with low complexity circuit configurations, low power receiver configurations, or communication systems implementing short duration training sequences. Moreover, some such techniques are often not suitable for providing high performance operation.
One technique for frequency synchronization, as shown in U.S. Pat. Nos. 8,605,830 and 8,948,320 (the disclosures of which are incorporated herein by reference), uses Fourier transforms to perform frequency synchronization in the frequency domain. However, implementation of Fourier transforms requires high computational complexity, resulting in the utilization of appreciable die size for the circuitry and relatively high power consumption. Accordingly, such techniques are not well suited for use in low power configurations (e.g., BLUETOOTH low energy (BLE) systems), configurations having relatively limited resources and relatively low complexity circuit configurations (e.g., some application specific integrated circuit (ASIC) or field programmable gate array (FPGA) circuit implementations), and the like.
Another technique, as shown in U.S. Pat. No. 7,039,132 (the disclosure of which is incorporated herein by reference), uses a clock recovery algorithm (e.g., an early-late gate algorithm) for time synchronization. Such clock recovery algorithms, however, generally require a long training sequence for the algorithm to converge. Accordingly, such techniques are not well suited for use in system configurations implementing short duration training sequences (e.g., BLE systems).
Yet another technique, as shown in U.S. Pat. No. 7,039,132 (the disclosure of which is incorporated herein by reference), uses a joint frequency and time synchronization technique. In this technique, the estimation of time synchronization is based on the result of frequency synchronization. Accordingly, the residue of the frequency estimation will degrade the performance of time synchronization. Such a technique thus does not provide high performance operation in many scenarios, particularly in situations where the frequency estimation is poor.